The long-term goal of this project is to elucidate the role of histone deacetylase 6 (HDAC6) in platinum resistance of non-small cell lung cancer (NSCLC). There are about 220,000 new lung cancer patients in the US annually which makes lung cancer the leading cause of cancer-related mortality in both men and women. Two first line regimens of lung cancer treatment are platinum with taxane and platinum with gemcitabine. One of the major obstacles of these treatments is that patients often develop resistance to platinum. HDACs are enzymes which remove acetyl moiety from histone and non-histone proteins, thereby being involved in a wide variety of biological processes including transcription regulation, cell growth/differentiation, apoptosis, etc. HDAC inhibitors now hold great promises in lung cancer treatment partly in that they are able to restore chemosensitivity. However, the mechanisms by which HDAC inhibitors enhance chemosensitivity are largely unknown. Our preliminary data has shown that one of the HDACs, termed HDAC6, acts as both a deacetylase and a ubiquitin E3 ligase to promote a key DNA mismatch repair (MMR) protein MSH2 deacetylation, polyubiquitination and degradation. Our results also indicate that deacetylation of MSH2 down-regulates MSH2 MMR activity. Moreover, we have shown that HDAC6 protein levels positively correlate with cisplatin resistance in a panel of 15 NSCLC cell lines. Given the fact that MSH2 serves as a sensor for cisplatin-induced DNA adducts and loss of MSH2 causes cisplatin resistance, we hypothesize that down-regulation of MSH2 protein level and its MMR activity governed by HDAC6-mediated deacetylation and ubiquitination contributes to platinum resistance. Therefore, in this proposal, we will first characterize HDAC6 ubiquitin E3 ligase activity towards MSH2 degradation and investigate how HDAC6-mediated deacetylation of MSH2 affects its MMR activity. We will then validate the role of HDAC6 in cisplatin resistance in a mouse xenograft model as well as in NSCLC patients' tissue specimens. The results of our research will identify HDAC6 as a novel therapeutic target in lung cancer treatment and show that HDAC6 and MSH2 may serve as biomarkers for platinum resistance in NSCLC patients. Our results will suggest the application of clinically relevant HDAC6-selective inhibitors which suppress HDAC6 deacetylase and E3 ligase activity for the treatment of NSCLC.